The Descent of Man

alists are practically guided by the following
considerations; namely, the amount of difference
between them, and whether such differences
relate to few or many points of structure, and
whether they are of physiological importance;
but more especially whether they are constant.
Constancy of character is what is chiefly valued
and sought for by naturalists. Whenever it can
be shewn, or rendered probable, that the forms
in question have remained distinct for a long
period, this becomes an argument of much
weight in favour of treating them as species.
Even a slight degree of sterility between any
two forms when first crossed, or in their offspring,
is generally considered as a decisive test
of their specific distinctness; and their continued
persistence without blending within the
same area, is usually accepted as sufficient evidence,
either of some degree of mutual sterility,
or in the case of animals of some mutual repugnance
to pairing.